Understanding the interactions of water
with a metal surface is fundamental in the
study of electrochemistry and catalysis. A
key question is how does water interact
with the surface---substrate/adsorbate, or
SA interactions---relative to its
interaction with
itself---adsorbate/adsorbate, or AA
interactions. The extent of clustering of
water on a metal surface indicates: (1)
its availability for participation in
surface reactions; and (2) the fraction of
contiguous, open surface sites available
for other reactions, whether or not they
involve water.

We have used density functional theory
(DFT) to examine water adsorption,
diffusion, and reaction in an effort to
understand how water participates in
heterogeneous catalytic and
electrocatalytic reactions.

Water Adsorption on Terrace, Step,
and Kink Sites

On platinum, the adsorption energy of
monomeric water (Fig. 1a) is 0.3 eV, about
the strength of a hydrogen bond.
Adsorption of additional water molecules
leads to growth of water clusters, as
water diffusion is facile on Pt. The
adsorption energy of dimer, trimer, (Fig.
1b,c) and larger clusters is approximately
0.5 eV, more in line with the heat of
sublimation of water. The increase in
adsorption energy with cluster size is
interpreted as the result of hydrogen
bonding among the water molecules.

For clusters on steps (Fig. 1f,i),
hydrogen bonding has little influence on
adsorption energy. Instead, the stronger
interactions of water with the steps is
the dominating factor in adsorption
energy. Although the adsorption energy is
still about 0.5 eV, it is due to
adsorption bonds formed between water and
the metal atoms of the steps. These
substrate/adsorbate bonds cause water to
adopt orientations unfavorable to hydrogen
bonding, so strong hydrogen bonding is not
possible.

These results provide the following
description of water adsorption on Pt.
Beginning with a clean surface, the first
water molecules adsorb as monomers that
rapidly migrate along the surface until
finding either a step or a kink at which
they adsorb strongly with little further
diffusion. The process continues until all
step and kink sites are filled, beyond
which adsorption occurs on the terrace
until complete coverage of the surface.
Desorption occurs in the reverse process.
Water molecules desorb from the terrace
first, followed by molecules that diffuse
from kinks and steps to the terrace and
then desorb from the terrace.

Diffusion

Diffusion of monomeric water from terrace
to step to kink is shown for a type B
step/kink in Fig. 2 (upper portion). Path
D1 is for terrace diffusion, D6
for terrace-step, D7 for along
the step, and D12 for
step-kink. The energy landscape for the
entire diffusion process, D1--D12,
is shown in the lower portion of Fig. 2
(dashed line) along with the analogous
process for the type A step (solid line).
The largest diffusion barrier of
0.22--0.26 eV is for the step-kink path, D12
(D10 for the type A step). Once
at the kink, water is strongly bound
and---with a diffusion barrier of 0.44
eV---will not diffuse away from the kink.
Other significant barriers for diffusion
from terrace-to-kink are terrace-terrace
(D1) at 0.20 eV, and step-step
for the type A step (D5) at
0.22 eV.

The calculations also reveal the
possibility of complex water molecule
motion along the terrace-terrace path,
shown in Fig. 3. In a simple translation,
water is parrallel to the surface for the
first portion of the reaction coordinate,
with the hydrogens lifting up from surface
parallel at the end of the trajectory. Two
other paths with similar diffusion
barriers are the roll, in which the
hydrogens roll over the top of the oxygen
atom, and the flip, in which the hydrogens
pass below the oxygen atom. While the
translation path has the lowest diffusion
barrier, our results do not allow the
other two to be excluded as possibilities.

Interconversion of
HCO and COH Intermediates

Carbon monoxide is a common intermediate
and, if it remains on the surface, a
poison in heterogeneous catalytic and
electrocatalytic reactions. In direct
methanol fuel cells, for example, CH3OH
successively dehydrogenates until forming
CO, which poisons the surface. In
developing CO-tolerant electrocatalysts,
it is necessary to understand the
mechanism for CO production.

Two intermediates of the CO formation
reation have been proposed: formyl (HC=O)
and hydroxymethylidyne (COH). Despite
numerous studies, these species have been
difficult to detect experimentally, due in
large part to their transient nature. We
have examined the adsorption and
activation energies of HCO and COH with
DFT to understand their stability with
respect to CO and to each other.

Figure 4 shows the energy landscape (top)
and configurations (bottom) of HCO, CO +
H, and COH on a Pt(111) surface. These
species were studied in the absence of
coadsorbed water (clean) and with one
water molecule. On the clean surface
(green line), CO + H is more stable than
either HCO (1.0 eV) or COH (0.48 eV).
Formyl has a modest activation energy
(0.33 eV) for reaction to CO + H, whereas
COH has a larger barrier (1.02 eV). The CO
+ H complex has activation barriers of at
least 1.33 eV, and so will not react to
either HCO or COH.

The presence of one water molecule alters
the HCO-COH surface chemistry
substantially. All three complexes; HCO,
CO + H, and COH are stabilized by water.
More importantly, the CO + H + H2O
complex has nearly the same adsorption
energy as COH + H2O with
essentially no activation barrier between
them (orange line). Thus, there is rapid
interconversion between the two complexes,
and an experimental measurement would be
unlikely to isolate the COH intermediate.
Direct conversion from HCO to COH (blue
line) has a moderate activation barrier.

Adsorption with a full water bilayer (not
shown) changes the surface chemistry
further. The COH intermediate is unstable
and dissociates to CO + H. The HCO species
is stable, however, and amenable to
detection. However, there is no
vibrational mode around 1700 cm-1
as would be expected for a carbonyl group.

The configurations in the lower portion
of Fig. 4 show that formyl adsorbs at a
bridge and interacts with one water
molecule at an atop site. In the CO + H +
H2O complex, CO adsorbs at a
hollow site, and water interacts with a
hydrogen atom at an atop site. The H2O-H
group is similar to a H3O+
hydronium ion. In the COH + H2O
complex, COH adsorbs at a hollow site, and
its interaction with water is strong
enough to displace water away from the
surface.

This work was supported by the Office of
Naval Research and the National Science
Foundation.

A portion of the research was performed
as part of an EMSL Scientific Grand
Challenge project at the W. R. Wiley
Environmental Molecular Sciences
Laboratory, a national scientific user
facility sponsored by the U.S. Department
of Energy’s Office of Biological and
Environmental Research and located at
Pacific Northwest National Laboratory.
PNNL is operated for the Department of
Energy by Battelle.

Last revised: December 11, 2012

Figure 1. Configuration of water monomer,
dimer, and trimer on the Pt(111) terrace,
(221) and (322) stepped, and (763) and
(854) kinked surfaces. All water
configurations are in their lowest energy
state. The solid lines show unit cells of
the surface. The dashed lines show unit
cells of the water layer. Type A surfaces
have (111)-oriented steps. Type B surfaces
have (100)-oriented steps.

Figure 2. (Top) Diffusion pathways for
water monomer on a type B step and kink
surface. (Bottom) Energy landscape for
water monomer diffusion from terrace to
step to kink sites of the type-A and
type-B surfaces.

Figure 3. Configuration of water monomer
in translational, rolling, and flipping
diffusion pathways. For clarity, the
reaction coordinate is not at the same
scale as the distance from the Pt surface.

Figure 4. (Top) Energy landscape for
HCO-COH interconversion through an
intermediate of CO + H. The green curve is
in the absence of water, and the blue and
orange curves are with one water molecule.
Values are in eV. (Bottom) Plan and
elevation views of HCO, CO + H, and COH
species with one water molecule. Blue is
oxygen, yellow is hydrogen, and orange is
carbon.